This invention relates to localized vacuum processing of a workpiece and, more particularly, to a localized vacuum envelope structure suitable for use in an electron beam lithography system.
Particle beam systems, such as ion implanters, electron beam lithography systems and ion beam lithography systems, require evacuation of the region along the path between the particle beam source and the workpiece being treated. The workpiece is introduced into the vacuum chamber, either directly or through an air lock. Such systems are highly complex and expensive. In addition, the vacuum pumping time causes a reduction in overall processing speed, a factor of utmost importance in a commercial semiconductor processing environment. These problems have been alleviated by the development of localized vacuum processing, as described hereinafter.
Certain particle beam processes require movement of the workpiece during processing. In electron beam lithography, microminiature patterns of extremely high accuracy are exposed on a workpiece. To achieve high accuracy, the electron beam deflection field is limited to an area much smaller than the area of the workpiece. Typically, an electron beam deflection field is a few millimeters on a side, while the workpiece, usually a semiconductor wafer or a mask plate, can be several inches in dimension. In order to expose the entire workpiece, precise positioning of the workpiece with respect to the electron beam is required.
Apparatus for vacuum porcessing in a localized region on the surface of a workpiece is disclosed in pending application Ser. No. 435,179, filed Oct. 19, 1982, and assigned to the assignee of the present invention. The vacuum processing apparatus includes an envelope which defines an internal vacuum processing zone. The tip of the envelope is positioned just above the surface of the workpiece and is spaced from the workpiece by a preselected gap. The tip of the envelope, together with the workpiece, forms a noncontacting graded vacuum seal between the internal vacuum processing zone and the ambient environment. The vacuum envelope provides a vacuum zone on the surface of the workpiece which is small in comparison with the size of the workpiece.
In order to treat the entire surface of the workpiece, the workpiece is mounted on a stage which is movable laterally with respect to the tip of the envelope. To achieve rapid processing in electron beam lithography systems, stage speeds of between and 1 and 10 cm/sec are typically employed. During this movement, the gap between the tip of the envelope and the workpiece must be dynamically controlled within specified limits. If the gap becomes too large, the vacuum in the vacuum zone is reduced, and the processing is interrupted until the required vacuum level can again be achieved. Electron beam columns used in lithography systems must be operated in the mid to high vacuum range. Typically, inadequate space is available to provide high conductance vacuum pumping. In such cases, the gap must be relatively small. When high conductance vacuum pumping can be provided, or lower pressure can be tolerated by the process, a larger gap is permitted. Conversely, the gap cannot become so small as to risk contact between the tip of the envelope and the workpiece. A workpiece, such as a semiconductor wafer, is extremely fragile and can be permanently damaged or broken by such contact. A charged particle beam system providing lateral movement of the workpiece during localized vacuum processing is disclosed in pending application Ser. No. 435,178, while a gap control system for localized vacuum processing is disclosed in pending application Ser. No. 435,177, both of which applications were filed Oct. 19, 1982, and assigned to the assignee of the present invention.
A critical factor in achieving practical localized vacuum processing is the structure of the vacuum envelope apparatus. The tip portion of the apparatus, which forms a noncontacting seal with the wafer, must be as small in area as practical, consistent with the size of the electron beam deflection field. This is necessary to permit processing near the edge of the wafer without losing vacuum as well as allowing the use of a vacuum chuck to hold the wafer rigid and flat. In the case of an electron beam lithography system, the vacuum envelope apparatus should have a small dimension in the direction along the axis of the beam. This requirement is necessary to avoid undue lengthening of the electron beam path and focal length of the final lens, both of which would cause an increased beam aberration. Furthermore, when the envelope apparatus has a small dimension along the path of the beam, the vacuum level in the processing zone is less critical. If the electron beam passes through a region of lower vacuum over a small percentage of its total path length, the effect on the beam is minimal. Another requirement is that a maximum amount of conductance be provided between the tip of the vacuum envelope and each of the vacuum pumps to which the apparatus is connected. This is difficult to achieve because of the small dimensions of the vacuum envelope, particularly the tip. The apparatus should not include structural members which obstruct gas flow, particularly in the region near the tip where gas flow is most restricted. A desirable feature of the envelope apparatus is that it provide magnetic shielding between the workpiece and the magnetic elements of the electron beam column. When such shielding is provided, the shield should be symmetrical about the beam axis to prevent deflection or aberration of the electron beam as it passes through the vacuum envelope. Finally, the vacuum envelope apparatus should be simple and inexpensive to construct.
It is a general object of the present invention to provide a novel localized vacuum envelope apparatus.
It is another object of the present invention to provide a localized vacuum envelope apparatus suitable for use in an electron beam lithography system.
It is yet another object of the present invention to provide a localized vacuum envelope apparatus which has a small dimension along a beam axis and which has a small area tip portion.
It is still another object of the present invention to provide a localized vacuum envelope apparatus with the capability to achieve a relatively high vacuum zone.